Bring the ginger beer, vodka and sugar to the boil in a small pan. Reduce the heat, simmer for approx. 2 mins., pour in the lime juice, pour into a stainless steel bowl, leave to cool. Cover the mixture and freeze for at least 8 hrs. or overnight. Remove, scrape the mixture with a fork until grainy, divide between glasses, cover and freeze for a further 30 mins. Sprinkle with the reserved lime zest.